You will also need to emerge either 'kde-base/kdesdk' or kde-base/kdesdk-scripts'.

If you emerged DBUS, CMAKE, QT or HAL you may skip those sections respectively.
Good luck!

Further dependencies are strigi, autmoc, akonado qimageblitz.and soprano. These packages can be found in the kdesupport module that is located in the kde svn repositories: svn://anonsvn.kde.org/home/kde/trunk/kdesupport. Since the module is pretty large and contains tons of things that you are probably not interested in, it is suggested that you selectively download the packages mentioned above:

Install via portage

kde4 now is in the official portage tree, although it is still hard masked (will probably change soon). kde3 and kde4 can be be both installed at the same time.

You will need to unmask them (see previous link) and then install it the usual way. As the future for kde in gentoo is to use "split" packages, the best bet is to go with
emerge -avD kde-base/kde-meta:kde-4